The present invention relates to a lighting system used in such equipment as imaging devices for recognizing and/or examining an object using a camera, and particularly to a lighting system having a linear light source and a reflector suitable for the light source.
In order to obtain an intended image using a camera, it is necessary to select an optimal lighting system suitable for that purpose. One practical example of such lighting systems is an LED lighting system including a linear light source having plural LEDs (light-emitting diodes) arranged in a row and a reflector (or reflectors) having a cylindrical reflective surface whose section is concave. The LED lighting system illuminates the longitudinal area of an object.
FIG. 7 is a section of a practically used LED lighting system, taken vertical to the longitudinal direction. In FIG. 7, numeral 1a denotes an LED packaged with resin or glass, and numeral 1b denotes a longitudinal board equipped with plural LEDs 1a. The LEDs 1a are arranged in a row on the board 1b at proper intervals. Thus, the LEDs 1a and the board 1b construct a linear light source 1. Numeral 2 denotes a reflector for reflecting a part of the light emitted from the linear light source 1. The optical axis of the linear light source 1 coincides with that of the reflector 2. Numeral 2a denotes a reflective surface of the reflector 2, whose section is concave. The reflective surface 2a is mirror-finished by a vapor deposition or plating of metal such as aluminum, or by plastering a tape or the like. The form of the concave section is generally an aspherical quadratic curve such as an ellipse or parabola. In the example of FIG. 7, the section is elliptical, and the linear light source 1 is disposed at or proximate to one (F1) of the two focuses of the ellipse. Numeral 3 denotes an irradiation plane disposed proximate to another focus F2 of the ellipse. Such an optical construction is based on the optical characteristic of an ellipse that all the light emitted from one focus is reflected by the elliptical surface and converges to another focus. The irradiation plane 3 is set to face the linear light source 1 and the reflector 2. When the concave section is designed parabolic and the light source is located at the focus of the parabola, the parabolic surface reflects the light and yields a parallel beam of light.
In the above LED lighting system, the light emitted from the linear light source 1 radially spreads like a solid angle around the optical axis. As the light spreads broader, the optical aberration of the LED 1a increases. Therefore, in general, the light within a preset solid angle around the optical axis is used as an effective light. In FIG. 7, the range of the effective light (effective emission angle) is shown as 2xcex8. Also, when an LED is designed for illuminating not a large area but a limited area of an object, the intensity of light emitted from the LED decreases as the spreading angle of light around the optical axis increases. Therefore, practically, the light utilized for illumination is mostly composed of a high intensity light within a narrow angle range around the optical axis, and a low intensity light surrounding the high intensity light is utilized merely supplementarily. As a result, the high intensity light directly illuminates the irradiation plane 3, and the surrounding low intensity light is first reflected by the reflector 2 and then illuminates the irradiation plane 3. In FIG. 7, the former is shown as a direct light 4a and the latter as a reflected light 4b. 
Generally, the direct light 4a is a diverging light, so that the direct light 4a illuminates a broadened area on the irradiation plane 3. Thus, it is only the light within a limited angle around the optical axis that effectively illuminates a desired area, while most of the direct light 4a illuminates outside of the desired area, thus being wasted. Further, the light illuminating the outside area is reflected by walls around and turns into a scattered light (which is called xe2x80x9cstray lightxe2x80x9d). The scattered light often badly influences the examination or the like, so that it must be eliminated by some means. Therefore, for example, a shielding plate having a narrow aperture is disposed close to the irradiation plane 3. In another example, the lens of the LED package is designed so that the emitted light converges only onto a desired area. The LED, however, lacks universal availability because it is designed for a particular distance between the linear light source 1 and the irradiation plane 3 and for a particular illumination area. When it is desired to locate the irradiation plane 3 as far from the linear light source 1 as possible, or when it is desired to reduce the illuminated area on the irradiation plane 3 as small as possible, the amount of wasted part of the direct light 4a increases. In this case, the amount of part of the direct light 4a reaching the irradiation plane 3 decreases, so that the luminance on the irradiation plane 3 decreases. Thus, in the illumination by the direct light 4a, some light wastage is inevitable.
The reflected light 4b, on the other hand, is a converging light, and all the light reflected by the reflective surface 2a converges to the irradiation plane 3. Therefore, in the illumination by the reflected light 4b, no light is wasted.
Thus, the conventional LED lighting system utilizes the direct light 4a that has a high intensity but is scattered and the reflected light 4b that is converged but has a low intensity, so that the efficiency is low.
The above problem might be solved by increasing the amount of the reflected light 4b while minimizing the amount of the wasted part of the direct light 4a. In order to attain that objective, however, it is necessary to greatly increase an effective diameter of the reflective surface 2a. Such a design is impractical because the reflector 2 would be so large that it would extend toward the irradiation plane.
One possible improvement to the prior art is to locate the linear light source 1 to face the reflector 2 so that the light emitted from the linear light source 1 around the optical axis is introduced to the reflector 2. FIG. 8 shows a section of an LED lighting system, taken vertical to the longitudinal direction, where the optical axis of the linear light source 1 coincides with that of the reflector 2 and the linear light source 1 is set to face the reflector 2. In this system, all the light spreading within a narrow angle around the optical axis and having a high intensity (i.e. the light propagating within the effective emission angle) is introduced to the reflector 2. However, part of the light within the effective emission angle, particularly the central part of the light including the optical axis and having a very high intensity, is obstructed by the linear light source 1 and/or the board 1b after being reflected by the reflector 2. As a result, that part of the light cannot reach the irradiation plane 3 and is wasted. In FIG. 8, the part around the optical axis where no hatching is done corresponds to the wasted part of the light. Thus, contrary to the expectation, the luminance on the irradiation plane 3 decreases, which prevents a practical use of the system.
A method of efficiently using the direct light 4a is known where a cylindrical lens is employed instead of the reflector. FIG. 9 shows a section of a lighting system using a cylindrical lens, taken vertical to the longitudinal direction. Numeral 5 denotes the cylindrical lens, which is formed to have an aspherical section so that it receives an effective light from the linear light source 1 within the angle 2xcex8 around the central axis and effectively converges the light to the irradiation plane 3. The production cost of the lens 5, however, is very high whether it is manufactured by a grinding of glass materials or by a molding of resin. In practice, the lens is required to be inexpensive to form. Therefore, only such lenses having a simple form and structure are practically available. Examples of such lenses are: a lens having a hemispherical section; a lens shaped like a rod; a cylindrical lens of a small diameter in section, etc.
Regarding other types of light sources constructed without LEDs, a cathode-ray tube is one of the most commonly known linear light sources. A typical example is a cold cathode-ray tube used in a backlight of a liquid crystal display. The cold cathode-ray tube employs a reflector to utilize as much light as possible. In general, a cathode-ray tube has a circular section taken vertical to the longitudinal direction of the tube, and the surface of the tube is processed to diffuse a light. By such a construction, light is irradiated from the whole surface of the tube in all directions with an equal intensity. There, the amount of light irradiated in the direction opposite to the irradiation plane 3 cannot be ignored. Therefore, it is necessary to introduce the light to the irradiation plane with the reflector. Thus, the reflector is indispensable.
Thus, the effect of the reflector differs depending on whether the light source is a cathode-ray tube or LEDs because of the difference in the state of light and the purpose of illumination. When the cathode-ray tube is used, the reflected light and the direct light are equally utilized. When, on the other hand, LEDs are used, the high intensity light within a narrow effective emission angle around the optical axis is mainly utilized as the direct light, whereas the surrounding low intensity light is less expected to be utilized. Thus, the conventional LED lighting system inefficiently wastes a considerable amount of light other than the light near the optical axis.
As described above, according to the prior art, a LED lighting system including a linear light source having LEDs arranged in a row and a reflector having a concave section and a cylindrical reflective surface is constructed so that the light emitted from the linear light source is separated into a direct light and a reflected light. The direct light is a diverged light, so that the illumination by the direct light covers a larger area than the desired area. Therefore, a considerable amount of the direct light having a high intensity is inevitably wasted, and the luminance on the irradiation plane cannot be increased. The luminance might be increased by using a lens, which, however, is very costly.
Besides, when only a desired area is to be illuminated, it is necessary to employ a shield, which consumes time and labor.
The present invention is designed to address the above problems. With this invention, all the effective light around the optical axis emitted from the linear light source is introduced to the reflector so that the illumination is carried out only with the reflected light. The reflected light is controlled to be a converging or parallel beam of light so that a desired area is illuminated and the luminance is increased.
Thus, in the first aspect of the present invention, a linear lighting system is proposed with a linear light source and a concave reflector having a cylindrical concave surface, which is characterized by the fact that a section of the concave surface, taken vertical to the linear light source, is part of an ellipse having a first focus at the linear light source and a second focus at an irradiation target point, where the part of the ellipse covers an effective emission angle of the linear light source and a reflected light produced at the part from the light within the effective emission angle is free of obstruction by the linear light source.
In a second aspect of the present invention, a linear lighting system is proposed with a linear light source and a concave reflector having a cylindrical concave surface, which is characterized by the fact that a section of the concave surface, taken vertical to the linear light source, is a part of a parabola having a focus at the linear light source, where the part of the parabola covers an effective emission angle of the linear light source and a reflected light produced at the part from the light within the effective emission angle is free of obstruction by the linear light source.
The xe2x80x9ccylindrical concave surfacexe2x80x9d is not restricted to the inside of a cylinder having geometrically circular section, but it includes any concave surface formed by a linear movement of an ellipse or parabola in a direction vertical to a plane on which the ellipse or parabola lies.
The light source and the irradiation target point need not exactly be at the focus. Even when they are slightly displaced from the focus, the object of the present invention can be attained. Accordingly, the term xe2x80x9cfocusxe2x80x9d includes not only the geometrical focus but also an area proximate to the geometrical focus. Further, the linear light source need not be straight. For example, it may be slightly curved around a point located on the side of the irradiation target point (or target line) farther or closer than the target point or line.